1. Technical Field
The present invention relates to a shock absorber for damping vibration transmitted to a vehicle according to a road state and, more particularly, to a shock absorber for controlling damping force characteristics by means of a hollow chamber formed inside a piston rod.
2. Description of the Related Art
Generally, a vehicle is provided with a suspension system for enhancing driving comfort by buffering impact or vibration transmitted to an axle from a road during driving. One of the components constituting the suspension system is a shock absorber. The shock absorber is typically disposed between the axle and a vehicle body. The shock absorber includes a cylinder and a piston rod reciprocating inside the cylinder. The cylinder is filled with an operating fluid, such as gas or oil, such that the operating fluid is moved by a piston valve secured to one end of the piston rod to generate a damping force.
A conventional shock absorber has a restriction in that it exhibits constant damping force characteristics with respect to variation of a road state or a driving posture of the vehicle. In other words, as determined in a manufacturing stage, a low (smooth) damping force characteristic can improve driving comfort of the vehicle, but cannot maintain a stable posture thereof. Conversely, a high (rigid) damping force characteristic can maintain the stable posture of the vehicle, but entails deterioration of the driving comfort. As such, the conventional shock absorber is incapable of controlling damping force characteristics in response to variation of the road state or the posture of the vehicle.
Accordingly, in order to solve the problem of such a conventional shock absorber, a shock absorber capable of providing variable damping force characteristics according to a displacement of the piston rod has been developed.
FIG. 1 is a partial cross-sectional view of a conventional shock absorber capable of providing variable damping force characteristics. Referring to FIG. 1, the shock absorber 10 includes a piston rod 14 connected to a vehicle body, and a cylinder 12 secured to an axle connected to wheels. The piston rod 14 reciprocates within the cylinder 12.
The piston rod 14 includes a piston valve 16 disposed at a lower end of the piston rod 14 to divide a space of the cylinder 12 into a rebound chamber RC and a compression chamber CC. The piston valve 16 is formed with a rebound passage 16a and a compression passage 16b through which the rebound chamber RC and the compression chamber CC fluidly communicate with each other. The shock absorber 10 further includes discs 18a and 18b disposed on upper and lower sides of the rebound passage 16a and the compression passage 16b to resiliently deform and generate a damping force according to movement of an operating fluid.
The piston rod 14 has a hollow chamber 20 formed therein. The hollow chamber 20 is provided with a free piston 22 that can move up and down and divides the hollow chamber 20 into an upper chamber 20a and a lower chamber 20b. An upper portion of the upper chamber 20a is in fluid communication with the rebound chamber RC through a through-hole 24, and a lower portion of the lower chamber 20b is in fluid communication with the compression chamber CC through a shaft hole 25.
As the piston rod 14 is slightly displaced, the free piston 22 is lifted or lowered, and the operating fluid flows into or from the hollow chamber 20 through the through-hole 24 or the shaft hole 25. As a result, the damping force characteristics of the shock absorber 10 are varied.
Referring to FIG. 2, Curve X1 indicates variation of the damping force of a general shock absorber which cannot control the damping force characteristics, and Curve X2 indicates variation of the damping force of the conventional shock absorber 10 capable of providing a responsive damping force. As shown in this plot, the conventional shock absorber 10 provides the damping force as indicated by Curve X2 according to the volume of the hollow chamber 20, that is, a range of motion of the free piston 22.
However, since the conventional shock absorber adopts a mechanical structure for buffering the impact or vibration, it is not effective in controlling the damping force and has a limited performance of varying the damping force.